A problem with a nitride semiconductor element, such as a gallium nitride (GaN) semiconductor element, is current collapse that decreases an ON-state current. Additionally, improvement of the breakdown voltage of the element, reduction in the number of cracks in the element, and suppression of warp of a silicon substrate on which the element is formed are desired. In order to improve the performance or alleviate these issues, a buffer layer between a silicon substrate and a channel portion of the element (a GaN layer) is employed.
The greater the thickness of the buffer layer, the more the breakdown voltage of the element is improved, and this improvement leads to suppressing of the current collapse phenomenon. However, it is easy for a pit (a cavity or a hole) or a crack to occur in the buffer layer, and the thicker the buffer layer, the greater the size of the pit or the crack. Therefore, when the buffer layer is thickened in order to improve the breakdown voltage, front surface states of the buffer layer or the channel portion deteriorate due to pits or cracks. Accordingly, a problem, such as an increase in leakage current, that degrades electric performance of the semiconductor element, occurs.